The invention relates generally to the field of networking. More specifically, it relates to the use of the Address Resolution Protocol (ARP) and to ensuring that only a single and consistent reply is generated in response to each ARP request.
The Address Resolution Protocol (ARP), used in TCP/IP networks such as the Internet, provides to requesting hosts a mapping between an IP address and a media access control (MAC) address. A host which needs to learn the MAC address for a given IP address broadcasts an ARP request containing the IP address to all routers and hosts in a network. The requests are received by adapters at the hosts; it is an adapter that owns an IP address and a corresponding MAC address. The requesting host learns the MAC address corresponding to an IP address by virtue of an ARP reply to an ARP request. An ARP reply is sent from the host that owns the corresponding adapter or, in some cases, an adapter is arranged to perform ARP processing and it responds to,ARP requests instead of the host. Such an adapter is called an offload adapter. In the remainder of this specification, host will be used to refer to both hosts that perform some kind of data processing in the traditional sense and to routers that route messages between networks or to nodes that perform both functions.
A host that owns multiple IP addresses that receives an ARP request will reply to the request only if the IP address in the request is the IP address of the adapter or if the adapter is explicitly configured to reply for the requested IP address.
A xe2x80x9crealxe2x80x9d IP address is one that is associated with a physical adapter. An adapter often supports thousands of network sessions with other hosts. If the adapter fails, all of the active sessions using the IP address associated with the adapter will also fail. Virtual IP addresses (VIPAS) were conceived to mitigate this problem. A VIPA is an IP address that is associated with a host, rather than with a physical adapter. Messages can be addressed to real IP addresses or to VIPAs. If a host contains multiple adapters, IP traffic addressed to a VIPA can be routed through any of the adapters. In this way, a host can provide fault tolerance after an adapter failure by routing the VIPA traffic over a different physical adapter. Virtual IP addressing is described in detail in U.S. pat. No. 5,923,854, the contents of which are incorporated by reference herein.
There are two types of physical adapters, a host adapter in which the host does all of the ARP request processing for the adapter and an offload adapter that does its own ARP request processing.
For ease of expression, in the remainder of this document letters such as A, B, C, X, etc. other than V designate physical adapters. The letter V denotes a virtual IP address. IP-A represents the IP address of adapter A; MAC-A represents the MAC address of the adapter A associated with IP-A. IP-V denotes the virtual IP address V. VIPA and IP-V actually refer to the same thing an IP address assigned to a host. Both of these designations are used interchangeably in this specification.
The traditional approach of ARP processing has a number of deficiencies. If adapters A and B are on the same physical network (i.e., all adapters on the network receive all ARP requests that any one of them receives) and both are owned by the same host, the host will not reply to ARP requests for IP-A received over adapter B. The host expects to reply to the request received over adapter A. This is a simple and effective way of preventing the generation of multiple replies to a single ARP request. However, it also means that no ARP reply will be generated if adapter A fails or is inactive. This means that adapters cannot serve as backups for one another. If a host owns IP-V and an ARP request for the MAC address assigned to IP-V arrives on adapter A, the host will not reply to the request, unless the owner has explicitly configured the system to do so. In the latter VIPA situation, if adapters A and B are on the same physical network, and A is assigned to IP-V, (explicitly configured to perform proxy ARP for IP address V), and adapter A fails, the host will no longer reply to ARP requests for V, even though it could send an ARP reply for V via adapter B. This often results in unsuccessful ARP requests.
The problem of providing backup adapters for offload adapters is even more difficult. For offload adapters, the host owning the offload adapter never sees an ARP request received over the offload adapter and the host likely has no knowledge of the MAC address of the offload adapter. If the offload adapter only replies to ARP requests containing its IP address, then it cannot provide any backup support for other adapters.
To address these limitations, a host could reply to any ARP request it receives over any adapter for any IP address owned by the host. However, when multiple adapters are on the same physical network, this will result in the host sending multiple ARP replies to a single ARP request and each will contain a different MAC address. This results in a flip-flopping of MAC addresses in the network for a single IP address. This, in turn, causes serious problems for network monitoring software. This flop-flopping of MAC addresses can also lead to odd traffic behavior and performance degradation.
To prevent multiple ARP replies when offload adapters are not involved, a host might implement a mechanism such that when the host first receives an ARP request over adapter A, it saves a timestamp and replies to the request. If within a short time it receives the same ARP request over adapter B, the host knows that an ARP response has recently been sent; so it ignores the ARP. Communication software in the Berkley Software Distribution uses this approach. A host might also implement such a technique to prevent multiple ARP replies for VIPAs. However, this timestamping solution still produces a flip-flopping of MAC addresses in a network. This is because there is a race as to which adapter A or B first receives an ARP request.
Therefore, there is a need for a solution that provides exactly one ARP reply with a consistent MAC address for any ARP request in an environment in which a host uses multiple adapters to address the same physical network, without the need for any user configuration.
Two embodiments are disclosed. The first embodiment is applicable to networks that do not contain VIPAs and offload adapters. The second implementation allows both types of physical adapters (host and offload) and VIPAs to coexist.
When an adapter (A) becomes active, the owning host sends an ARP advertisement into the network over adapter A that associates the MAC address for adapter A (MAC-A) with an IP address (IP-A). This advertisement is received by all hosts in the network and they update their ARP cache table to map IP-A to MAC-A accordingly. If the advertisement is also received at the sending host over a different adapter B, then the host knows that adapter B is on in the same physical network as adapter A. Therefore, B can be designated as a backup adapter for A and A can be designated as backup adapter for B. The host maintains a backup adapter field for each adapter owned by the host where this information is maintained. When the host discovers that adapter B is in the same network as adapter A, it queries the backup adapter field. If no backup adapter has been designated for A, then the host sets B as the backup adapter for A. Likewise, the host queries the backup adapter field for adapter B and sets A as the backup adapter for B if no backup adapter has already been designated.
If adapter A fails or becomes inactive, the host resets the backup adapter field for any adapter it owns for which A is marked as the backup adapter. If a backup adapter B has been designated for A, the owning host also sends an ARP advertisement associating MAC-B with IP-A. This advertisement causes each host in the network to update their ARP cache table to map IP-A to MAC-B. This allows network connections to IP-A originally served via adapter A to continue non-disruptively over adapter B and it also provides access to the host for subsequent new connections. Whenever the host receives an ARP request for IP-A on adapter B, the host replies to the request with MAC-B.
When adapter A later becomes active, the host sends a gratuitous ARP advertisement that maps IP-A to MAC-A. This allows adapter A to re-assume responsibility for responding to ARP requests for IP-A.
The first embodiment depends on the host receiving ARP requests to determine what networks its adapters are on. Therefore, it does not function properly in networks that include offload adapters, because it does not receive ARP requests for these adapters. The adapters handle the ARP processing.
To solve the problems for offload adapters, the invention uses a different technique to determine what adapters of a host are on the same networks. This technique also works for host adapters and is further arranged to accommodate VIPAs as well. In each host, the first adapter A to become active is designated as being in a first physical network (PNET1). The identification assigned to the network is arbitrary. It is only necessary to differentiate each separate network for the benefit of the host. For each subsequent adapter B to become active on a host, the host sends a packet over one of the adapters of each network already known to the host with a hop count of one. In the case of the second adapter to become active, the packet would be sent over adapter A. In the preferred embodiment the packet is an ICMP (Internet Control Message Protocol) echo request, although it could be any type of packet that allows a hop count of one. The hop count of one ensures that the packet will not be forwarded off of the network by a network router. The packet will be received by adapter B only if A and B are in the same physical network on which it is sent. Therefore, if the packet is received over adapter B, it is known that adapters A and B are in the same physical network. If this occurs, adapter B is marked as being in the same network PNET1 as adapter A. If the packet is not received over adapter B, as evidenced by an eventual timeout function, then it is known that adapter B is in a different physical network as A. In this event, adapter B is marked as being in a new network PNET2. In general, the algorithm to determine in which network each offload adapter resides can be stated as follows. When an adapter becomes active at a host, send a data packet with a hop count of one over one adapter that resides in each different physical network known by the host and, if the packet returns on a different adapter, add the newly active adapter to the same physical network to which the receiving adapter belongs. If the packet does not return on a different adapter, create a new physical network at the host and add the new adapter to that network. If the new adapter also happens to be an offload adapter, then the host registers the IP address in the adapter. This causes the adapter to associate the IP address with the adapter MAC address known to the adapter and also to transmit an ARP advertisement into the network. If the new adapter is a host adapter, the host sends the advertisement itself. To handle VIPAs, after the first adapter on a host becomes active, it is initially marked as owning ARP responsibility for all virtual IP addresses owned by the host for the physical network in which the adapter is located.
When an adapter A becomes inactive in the second embodiment, if there are other active adapters in the physical network to which A belongs, then one of the remaining adapters B in that physical network is designated to have the responsibility for replying to ARP requests for IP-A. If B is a host adapter, the host sends a gratuitous ARP advertisement request mapping IP-A with MAC-B. If B is an offload adapter, the host registers IP-A in adapter B. This causes adapter B to associate its MAC address MAC-B with IP-A; the offload adapter also sends the gratuitous ARP advertisement. In either case, other network hosts update their ARP caches so that connections to IP-A will continue non-disruptively over adapter B. The host next determines if adapter A is marked as owning responsibility for VIPAs. If it is, then that marking is removed and adapter B is marked as owning VIPA ARP responsibility for that physical network. If B is a host adapter, then for each VIPA known to the host, it sends a gratuitous ARP advertisement into the network associating IP-V with MAC-B. If B is an offload adapter, the host registers IP-V with the adapter for each known VIPA and the adapter sends the advertisements into the network. Thereafter, when the host or an offload adapter on the host receives an ARP request for A or V, the host or offload adapter replies to it with the MAC address of B. When adapter A again becomes active, adapter A will re-assume ownership of ARP replies for IP address A.
The claims of this application are directed to the second embodiment wherein only host adapters are considered.